Biomass gasification systems for residential application: An integrated simulation approach

Applied Thermal Engineering 71 (2014) 152e160

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Biomass gasification systems for residential application: An integrated simulation approach Dario Prando a, *, Francesco Patuzzi a, Giovanni Pernigotto a, b, Andrea Gasparella a, Marco Baratieri a a b


5, 39100 Bolzano, Italy Faculty of Science and Technology, Free University of Bolzano, Piazza Universita Department of Management and Engineering, University of Padova, Stradella San Nicola 3, 36100 Vicenza, Italy

h i g h l i g h t s
CHP system based on biomass gasification to meet household energy demand is studied.
Influence of CHP size and operation time on energy performance has been analysed.
The economic performance of a CHP systems have been investigated.
Effect of subsidisation regime on the economic analysis has been examined.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 March 2014 Accepted 22 June 2014 Available online 30 June 2014

The energy policy of the European member States is promoting high-efficiency cogeneration systems by means of the European directive 2012/27/EU. Particular facilitations have been implemented for the small-scale and micro-cogeneration units. Furthermore, the directive 2010/31/EU promotes the improvement of energy performance of buildings and use of energy from renewable sources for the building sector. In this scenario, systems based on gasification are considered a promising technological solution when dealing with biomass and small scale systems. In this paper, an integrated approach has been implemented to assess the energy performance of combined heat and power (CHP) systems based on biomass gasification and installed in residential blocks. The space-heating loads of the considered building configurations have been simulated by means of EnergyPlus. The heat load for domestic hot water demand has been calculated according to the average daily profiles suggested by the Italian and European technical standards. The efficiency of the whole CHP system has been evaluated supplementing the simulation of the gasification stage with the energy balance of the cogeneration set (i.e., internal combustion engine) and implementing the developed routines in the Matlab-Simulink environment. The developed model has been used to evaluate the primary energy saving (PES) of the CHP system compared to a reference case of separate production of heat and power. Economic analyses are performed either with or without subsidizations for the generated electricity. The results highlight the capability of the integrated approach to estimate both energy and economic performances of CHP systems applied to the residential context. Furthermore, the importance of the generated heat valorisation and the proper system sizing have been discussed. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Biomass gasification Internal combustion engine Combined heat and power Residential buildings Primary energy saving Economic analysis

1. Introduction In Europe, energy use in buildings accounts for around 40% of the total energy consumption. Therefore, reduction of energy

* Corresponding author. Tel.: þ39 0471 017615; fax: þ39 0471 017009. E-mail addresses: [email protected], [email protected] (D. Prando). http://dx.doi.org/10.1016/j.applthermaleng.2014.06.043 1359-4311/© 2014 Elsevier Ltd. All rights reserved.

consumption and use of energy from renewable sources in the building sector are important measures to meet the climate and energy targets set by the European Union [1,2]. Furthermore, among the different renewable energy sources, biomass could provide a considerable contribution to develop distributed generation systems and offset fossil fuels consumption [3,4]. There is also an increasing interest in the cogenerative production of heat and power because it can increase the overall efficiency of energy systems and reduce the global CO2 emissions [5,6]. PES of CHP systems

D. Prando et al. / Applied Thermal Engineering 71 (2014) 152e160

is extensively analysed in literature; Pohl et al. [7] analysed the influence of plant-side and demand-side characteristics, Rosato et al. [8] investigated the effects of transient operation of CHP systems and Angrisani et al. [9] assessed the advantages of thermal load sharing by means of a thermal micro-grid. Fumo et al. [10] emphasised the importance to evaluate the primary energy saving of a CHP system design before the economic viability. Boschiero et al. [11], Pagliarini et al. [12] and Piacentino et al. [13] highlighted the benefits related with the implementation of trigeneration systems and discussed the importance of proper policies supporting this technology. The European Parliament, with the directive 2012/27/EU, promotes the cogeneration based on useful demand for heating or cooling [14,15]. The most common technology to convert biomass into heat and power is based on biomass boiler coupled with Organic Rankine Cycle (ORC). For sizes larger than 500 kWel, the ORC technology is reliable and highly standardized with reasonable investment and operational cost. According to Quoilin et al. [16] and Rentizelas et al. [17], at these sizes, ORC systems can reach net electrical efficiencies up to 17%. According to Dong et al. [18] and Maraver et al. [19], the ORC technology is less attractive for small scale applications because economies of scale penalise both the efficiency and the specific cost. According to Joelsson et al. [20], CHP systems based on biomass gasification represent a promising technological solution and a possible alternative to conventional biomass cogeneration systems. The integration of biomass gasification with high efficiency power generation systems can define competitive scenarios, but it is an investment option with strong dependency on the support policies [21,22]. The high efficiency that can be reached by gasification systems enables the development of energy models based on distributed generation at sizes that have not been sufficiently efficient until now. Moreover, the decentralization of the energy production would lead to various benefits, e.g., reduction of the transmission and distribution losses, exploitation of the local resources and reduction of the energy used for the transportation of the feedstock [17,23]. The research concerning the coupling of biomass gasification with traditional power generators (gas engines and gas turbines) is well documented by Dong et al. [19] for the small-scale and microscale systems, and by Fagbenle et al. [24] for the large-scale systems. Benefits and obstacles of innovative generation systems, such as fuel cells, are described by Baratieri et al. [25] and Tommasi et al. [26]. The implementation of gasification-based CHP for rural areas, where the electrical network does not exist, is discussed by Coronado et al. [27] and Lee et al. [28]. Besides the development of mathematical models for a given biomass processing systems, the simulation of a complete energy conversion plant is usually carried out through models that offer advantages in the evaluation of process performance in different operating conditions [29]. However, the evaluation of the overall system performance still requires further development to evaluate each technology as integrated into the complete chain (biomass pre-processing, biomass energy conversion, energy distribution, final use). Moreover, small-scale and micro-scale CHP systems based on biomass are not a completely consolidated technology and some technical and economic issues have still to be addressed [19]. Nevertheless, micro CHP systems based on gasification have been recently introduced in the market, and they can be used for small industrial application or building with several users (e.g., hotels) [30,31]. At the current stage, the management of gasification startstop operations are quite complex, requiring a continuous operation of the system and subsequent dissipation of excess heat when there is no specific demand.

153

In this work, an assessment of the energy performance of a biomass micro-CHP system for residential application has been carried out by means of an integrated approach. The power plant is based on biomass gasification, and the end-user consists of some blocks of flats. The overall energy performance has been evaluated for various contexts; different size and operational time of the generation system, and different building configurations. The energetic assessment has been conducted for the complete chain from the production stage to the final user and to this purpose a multistage model has been developed and applied. The evaluation has been performed in terms of PES with respect to a reference conventional technology of separate production of heat and power based on biomass. In addition, the economic analysis of a test gasification system of 30 kWel has been performed either with or without subsidizations for the generated electricity. 2. Methodology 2.1. Building stock characterisation In the application of the integrated approach proposed in this work, four buildings close each other and heat distribution network e supplied by a common central system e have been modelled. The geometrical and thermo-physical characteristics are the same for the four buildings, but different configurations have been studied by varying some features (i.e., insulation level, kind of glazing and windows orientation), in order to evaluate different heat load profiles. The selected cases are designed to allow correlating their main features to the performance (both energetic and economic) of the cogeneration system and are not chosen to represent the Italian building stock. Each building consists of ten floors, each one with 100 m2 of floor area without internal partitions and 3 m of internal height. The ratio between the dispersing surface area and the net volume of each building (S/V) is 0.47 m1. The vertical surfaces of the envelope face the cardinal points. For each building, the floors of the flat at the ground level is considered in thermal contact with an unheated highly ventilated basement, and hence modelled as exposed to the external environment, without solar and infrared radiations exchanged with the sky dome. The opaque elements have a simplified two-layer structure, with a 20 cm thick layer of massive clay block (thermal conductivity 0.25 W m1 K1, density 893 kg m3, specific heat capacity 840 J kg1 K1) on the internal side, and an insulating polystyrene layer (thermal conductivity 0.04 W m1 K1, density 40 kg m3, specific heat capacity 1470 J kg1 K1) on the external side, with variable thicknesses. The effect of the thermal bridges has been neglected. All the surfaces, both the internal and the external sides, have solar radiation absorption coefficient of 0.3, with the exception of the internal floors and the external roof that have a coefficient of 0.6. The ratio between the area of the glazings and the internal floor is 11.67%. The windows are double-glazed with a thermal transmittance of the glazing area equal to 1.1 W m2 K1. The frame area is 20% of the whole window area (14.56 m2) and its thermal transmittance is 1.2 W m2 K1. The ventilation rate is constant and equal to 0.3 ACH, in accordance with the Italian technical specification UNI/TS 11300-1:2008 [32]. The internal gains have been assumed constant and equal to 4 W m2, half radiant and half convective [32]. The heating air temperature setpoint has been set to 20  C in accordance with the UNI/TS 113001:2008 prescriptions for residential buildings [32]. The residential buildings are located in Milan with 2404 Heating Degree Days (HDD) with respect to a base temperature of 20  C. This location, in accordance with the Italian climatic classification, refers to the climatic zone E (2100 HDD e 3000 HDD) that is the most characteristic and populated in northern Italy.

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Different building configurations have been evaluated for the buildings previously described considering alternative options for some characteristics as follows. Three possible thicknesses have been considered for the insulating polystyrene layer of the opaque elements; 0, 5 and 15 cm, which provide average thermal transmittances respectively equal to 1.03, 0.45 and 0.21 W m2 K1 (i.e., uninsulated, poorly insulated and well-insulated buildings). As concerns the transparent components, which are all positioned on a single façade, two options have been analysed; east or west oriented. These two possibilities allow having the maximum solar gain in the morning (east oriented) rather than in the afternoon (west oriented). Finally, two glazings with different solar heat gain coefficients, SHGC, (0.608 and 0.352) have been considered in order to assess two different levels of solar heat gains. Combining the alternatives related to the insulation of the opaque components, the positions of the transparent ones and the glazings SHGC, 12 different building configurations have been obtained. The simulation of the building dynamic behaviour has been conducted by means of EnergyPlus 7.1, a validated software for the simulation of real buildings [33]. As comparison of the considered alternatives, Fig. 1 shows the space daily heating load profiles for the 12 building configurations in January (winter season) and April (middle season). As regards the thermal losses of the delivery section, the simplified approach of seasonal values e proposed by the technical specification UNI TS 11300-2:2008 [34] e has been adopted. Concerning the space heating system, the emission and distribution efficiencies depend on the envelope insulation level in agreement with the technical specification UNI TS 11300-2:2008 [34]. The heating system is based on radiators; the emission efficiency is 0.90 for the cases without thermal insulation, 0.93 for those ones with 5 cm of insulation and 0.95 for those ones with 15 cm of insulation.

Fig. 1. Average space daily heating load profile for four buildings (each with ten apartments) for an average day of January (a) and April (b). Building configurations: window orientation (east, west), insulation thickness (0, 5 and 15), SHCG (high, low).

The distribution efficiencies are equal to 0.97, 0.98 or 0.99, respectively for 0 cm, 5 cm and 15 cm of insulation. The control efficiency can be considered 0.94 for all cases and corresponds to on/off temperature control for each thermal zone of the building. The domestic hot water demand (DHW) has been determined considering an average daily tapping pattern for a family with shower use in accordance with the EN 15316-3-1:2008 [35]. The contemporary DHW demand of 4 buildings (10 flats per building) has been determined in accordance with the UNI 9182:2010 [36]. Fig. 2 shows the daily DHW demand considered in the integrated building-system analysis. The domestic hot water system is characterized by an emission efficiency of 0.95 because no devices for the control of supply are considered in the system [34]. The distribution thermal loss has been considered negligible since the distribution pipes are well insulated. 2.2. Power plant layout The power plant has been simulated with a multistage model by means of Matlab-Simulink environment. The gasification stage model has been developed and validated by Baratieri et al. [37], and it is based on the thermochemical equilibrium using the Cantera solver and the Gri-Mech thermodynamic properties [38]. The developed multistage model has a general predictive capability that allows defining both electrical and thermal energy production depending on the considered operating conditions. The power plant layout considered in this work (Fig. 3) is based on the generation and use of the producer gas to generate electrical and thermal power. The producer gas generation section has been modelled as a downdraft fixed bed gasifier operating in ideal conditions, which can simulate different equivalence ratios (ER) e i.e., the actual air-fuel ratio divided by the stoichiometric air-fuel ratio e and therefore both pyrolysis (ER ¼ 0) and air gasification processes (ER > 0). The simulated overall gasification process can be endothermic or exothermic depending on the equivalence ratio. For an endothermic process, the heat is provided through a burner fed by a producer gas spilling. For an exothermic process, the heat is simply discharged. The pressure inside the gasifier is considered to be atmospheric. Before feeding the CHP, the producer gas is piped through heat recovery and clean-up sections. Heat exchangers are assumed to be adiabatic with the producer gas being cooled from the gasification temperature to 150  C. This temperature has been chosen to minimize tar condensation that would clog up the heat exchanger. The clean-up section can be considered a condenser where tar and water vapour are condensed and cooled to 25  C. The share of heat exchanged in the clean-up section is discharged to the environment due to heat losses to the surroundings. Pressure losses due to the ancillary equipment and to the filters have not been

Fig. 2. Average daily tapping pattern for 40 families with shower use.

D. Prando et al. / Applied Thermal Engineering 71 (2014) 152e160

155

Fig. 3. Schematic diagram of the power plant layout.

considered. Electricity consumption of the auxiliaries has been considered as much as 17% of the gross electric production [39]. In this work, poplar wood has been considered as feedstock for the gasification process due to its availability in northern Italy. Its elemental composition, moisture content and heating value (Table 1) have been employed as inputs for the thermochemical equilibrium model. The feedstock characteristics are reported on as received (ar) basis, i.e., considering the water content in the feedstock mass [40]. The producer gas coming from biomass gasification is exploited in an internal combustion engine (ICE) based on Otto cycle. As for the gasifier, also the processes in the CHP has been modelled at thermodynamic equilibrium in Matlab-Simulink environment. The Otto cycle has been modelled as a fueleair cycle. Combustive air and producer gas are compressed in isentropic conditions according to a specified volume ratio (v1/v2). Then complete combustion occurs at constant volume, followed by an isentropic expansion of exhaust gases and a discharge of exhaust gases at constant volume. The efficiency of the real cycle has been calculated considering a ratio between the useful work and the work of the air-fuel cycle of 70% [41]; the complementary share of input energy has been considered to be recovered both through the cylinder coolant and the exhaust gas cooling (see Fig. 3). The exhaust gases from CHP are processed by means of a heat exchanger and a clean-up system with the same features previously described for the gasification section. Purified exhaust gas is then ready to be heated up to 140  C through a heat exchanger and piped to the chimney. Nowadays, the clean-up section is not implemented for small scale plant, such as the one evaluated in this paper. However, it has been considered as precautionary measure in a future perspective of extensive development of decentralised generation in residential areas. The Otto cycle is generally optimized in order to operate with gasoline which has different properties with respect to the producer gas. If compared to gasoline, producer gas has a higher auto-ignition temperature, hence it allows the adoption of higher volumetric ratio for the engine. In this work, volume ratio has been fixed to 15 [42]. The conversion efficiency from mechanical to electrical power is assumed as 94% [43]. Thermal power is recovered by means of adiabatic heat exchangers, from both the gasification and CHP sections. The power-to-heat ratio has been calculated as electrical power divided by the thermal power of the whole power plant. Furthermore, electrical and thermal power have been computed to evaluate the electrical (1) and thermal (2) efficiency. Table 1 Characteristics of poplar wood. Moisture

ash

C

H

O

N

LHV [MJ/kgar]

Ref.

1.1

41.2

5.0

37.2

0.4

14.936

[40]

[%wtar] 15.0

hel ¼

Pel LHVbiom $Qm;biom

(1)

hth ¼

Pth LHVbiom $Qm;biom

(2)

where LHVbiom is the lower heating value of the biomass and Qm,biom is the mass flow rate of biomass, Pel is the CHP electrical power and Pth is the thermal power recovered from both the gasifier and CHP. Gasification temperature and equivalence ratio have been optimised to reach the highest possible electrical efficiency since electricity is a form of energy of high quality. The gasification temperature has been evaluated between 500  C and 1000  C, with a step of 25  C, and the equivalence ratio has been evaluated between 0.0 and 0.6, with a step of 0.025. The efficiencies obtained in this optimization procedure have been used for the integrated analysis of buildings and biomass gasification system. 2.3. Integrated analysis of building and CHP system In Italy, the gasification systems usually run without taking care of the dissipated heat due to the feed-in tariff on the electrical production (EUR/kWhel) that is more attractive than heat valorisation. The incentive is paid by the electrical services management company (i.e., GSE e Gestore Servizi Elettrici), which is the Stateowned company that operates the mechanisms of promotion and support of the renewable energy sources in Italy [39]. At the current technical stage, the gasifier systems usually require complex procedures to reach a steady state operation; a partial or on/off operational mode is considered not yet feasible. According to these considerations, in this work, the power plant is supposed to run continuously for a period centred on the coldest day of the year. The electricity production is entirely provided to the electrical grid that acts as an ideal storage. In contrast, only a fraction of the thermal production is useful; when buildings have reduced or no heat demand, thermal power has to be dissipated. Nevertheless, a thermal energy storage (TES) could enhance the useful heat, even if the system is not supposed to operate on/off. A preliminary analysis has shown that implementation of TES e with volume up to 10,000 litres e would enable exploiting an additional share that is smaller than 2% of the heat produced by CHP system. In this work, a conservative assessment has been carried out neglecting the contribution of TES. The primary energy saving has been calculated to evaluate the advantage in adopting a cogeneration system instead of the separate production of heat and power. In agreement with the Directive 2004/8/EC [14], PES has been calculated as:

PES ¼ 1  CHP Hh Ref Hh

1 Eh þ CHP Ref Eh

(3)

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where CHP Hh is the thermal efficiency defined as annual useful heat output divided by the CHP fuel input, CHP Eh is the electrical efficiency defined as annual electricity divided by the CHP fuel input, Ref Hh is the efficiency reference value for separate heat production, Ref Eh is the efficiency reference value for separate electricity production. The reference efficiencies depend on the construction year of the power plant and the fuel supply; the values are reported in the Italian decree of the 4th august 2011 [44]. Considering the plant construction in 2013 and wood as fuel supply, Ref Eh is equal to 0.33 and Ref Hh is equal to 0.86. In accordance with the Italian decree, Ref Eh has to be corrected considering the average ambient temperature (i.e. 11.3  C for the considered application) and the connection line voltage (i.e. 400 V for the considered application); the resulting value of Ref Eh is 0.315. PES values have been evaluated for all building configurations, for power plant sizes between 10 kWel and 100 kWel, with a step of 5 kWel, and for operational period durations comprised between 1560 h (65 days) and 8760 h (365 days), in steps of 400 h.

Table 2 Parameters for the economic analysis. Parameter

Value

Ref.

IC, gasifier þ engine [EUR/kWel] IC, engine [EUR/kWel] Maintenance cost [EUR/kWhel] Biomass cost [EUR/t] Feed-in tariff [EUR/kWhel] Cogeneration bonus [EUR/kWhel] Heat valorization [EUR/kWhth] Electricity revenue (GSE)c [EUR/kWhel] Real interest rate [%]

4500 500 0.050 165 0.229 0.040 0.057 0.120 3.00

a a a b

[39] [39] [47] [45] e

a M. Prussi, personal communications, October 2013, C.R.E.A.R. University of Florence. b V. Francescato, personal communications, September 2013, A.I.E.L. c Alternative to comprehensive incentive and cogeneration bonus.

on both net present values (NPV) and AW has been presented for the representative power plant configuration.

2.4. Economic analysis

3. Results and discussion

As described in the results section, PES analysis has highlighted some building configurations more relevant than others. These configurations have been considered for further study, such as economic analysis. Firstly, a differential cash flow between the CHP plant and a traditional non-condensing gas boiler (reference case) has been evaluated in subsidization regime. The traditional gas boiler has also been considered in the CHP scenario as back-up of the CHP system. The subsidisation is mainly the feed-in tariff paid by GSE on the net electricity delivered to the grid; for the considered power plant, the tariff is guaranteed for 20 years. Moreover, a bonus per cogenerative electricity is paid by GSE if the CHP system has high efficiency; for a power plant smaller than 1 MWel, the PES has to be higher than zero. The cogenerative electricity depends on the useful heat from cogeneration, and it is calculated according to the Directive 2004/8/EC [14]. The electricity purchased by the users has not been considered in the analysis since, in both scenarios, it is completely drawn from the national grid. This is a common practice since the incentive tariff is higher than the electricity price, and the incentive is not paid for the self-consumption electricity share. In addition, a second differential cash flow has been determined without subsidization. In this case, GSE offers a “simplified purchase & resale arrangements” to trade the electricity produced with renewable sources [45]. The economic analysis is performed by calculating two indexes: the discounted payback time (PBT) e to evaluate the time required to recover the investment cost e and the annual worth (AW) e to estimate the annual revenue of owning and operating an asset over its entire lifespan. The gasifier lifespan (80,000 h) has been adopted as a reference for the length of the investment analysis. As regards the engine, a lifespan of 40,000 h has been used. The considered real interest rate is 3% [46]. Table 2 reports the investment costs (IC), the operational costs, the feed-in tariff, the electricity price guaranteed by GSE for the “simplified purchase & resale arrangements” and the revenues (VAT and other taxes excluded) required to perform an economic analysis. The economic valorisation of the heat due to the CHP has been calculated considering the natural gas savings of the back-up noncondensing boiler. Considering a seasonal efficiency of the back-up boiler of 93%, natural gas LHV of 34.9 MJ Sm3 and natural gas cost for residential use equals to 0.51 EUR/Sm3, the thermal power is valorised at 0.057 EUR/kWhth [21,33]. Finally, a sensitivity analysis of the main parameters (i.e. IC, biomass cost, feed-in tariff, cogeneration bonus, heat valorisation)

3.1. Gasification section The results of the optimization procedure e carried out on the gasification system to maximise the electrical efficiency e have been reported in Fig. 4. The iso-efficiency curves have been plotted interpolating the points in which the electrical efficiency has been calculated, as explained in methods section. The maximum electrical power has been obtained for a gasification temperature of 800  C and ER of 0.1. In this configuration, the CHP electrical efficiency results 0.23 and the CHP global efficiency results 0.85. The global efficiency has been computed as the sum of the electrical and the thermal efficiency. The output power-to-heat ratio corresponds to 0.375. The energetic optimization of the producer gas generation section corresponds to the complete conversion of carbon. A share of 26% of the producer gas is used to feed the heater that provides heat to the gasifier (endothermic process). The total thermal power is recovered by the heat recovery section of the gasifier (15%) and by the ICE section (85%). 3.2. Power plant and buildings The results of PES analysis for all the considered building configurations (12 alternatives) are shown in Fig. 5. The graph reports the curves corresponding to a PES equal to zero; i.e., the CHP system and the separate production of heat and power have the same consumption of fuel. The iso-PES curves have been plotted

Fig. 4. Electrical efficiency of the whole power plant layout depending on the equivalence ratio and the gasification temperature.

D. Prando et al. / Applied Thermal Engineering 71 (2014) 152e160

Fig. 5. Curves of PES equal to zero depending on the power plant size and the operational time for all the considered building configurations.

interpolating the points in which the PES index has been calculated, as explained in methods section. The curves show clusters for the building configurations with the same thermal insulation of the opaque envelope. The orientation and SHGC of the windows partially affect the PES, slightly shifting the curves inside these clusters. For this reason, the results of the energetic and economic analysis have been shown in detail for three building configurations with 0, 5, 15 cm of thermal insulation of the opaque components and high SHGC west-oriented windows, for which the resulting curves appear in the middle of the clusters. Fig. 6 shows the detailed results of PES analysis for the three building configurations with different thermal insulation of the opaque components. Positive areas of PES have been detected for the three building configurations. PES index is related to the exploitation of the produced heat, therefore, the less heat is discharged the larger is PES. Short operational time and small plant size usually allow higher primary energy saving values, but there are some limitations that have to be considered. First of all, plants smaller than 30 kWel are not commercially available. Furthermore, lower operational times (e.g., shorter than 4000 h) could lead to high payback periods. Fig. 7 compares the thermal load of the buildings without envelope insulation and the heat produced by a power plant for the entire year. As previously mentioned, a higher thermal power production could be entirely exploited only during the coldest

insulation: 0 cm

100

157

months; the produced heat would be discharged for most of the operational time. Considering the smallest possible plant size (30 kWel), Fig. 8 shows the discounted cash flow for the power plant running for 2,000, 4000 and 6000 h and considering scenarios either with or without subsidizations. The economic analysis has been performed for the three analysed building configurations (0, 5, 15 cm of the envelope insulation). Considering the incentive regime, power plants with low operational time, e.g. less than 4000 h, move to high payback time due to a long amortisation schedule. Moreover, the feed-in tariff is guaranteed for 20 years, afterwards the revenues have the same order of magnitude of the operational costs with a resulting profit close to zero or even negative. The discounted cash flow, corresponding to the scenario without incentives, shows that the gasification system profitability strictly depends on the incentive paid for the electricity production. Fig. 9 compares the revenues of the considered system, and it highlights the importance of the heat valorisation, in particular for the scenario without incentive. With the current market prices, the revenue coming from a full utilization of the generated heat would be higher than the revenue for the electricity trade (i.e., simplified purchase & resale arrangements). The revenue coming from heat valorisation has been displayed in EUR/kWhel for comparison purpose, but it can be converted in EUR/kWhth multiplying the power-to-heat ratio (i.e., 0.375). Fig. 10 shows the payback time and the annual worth for the three analysed building configurations. The choice of the optimal operational time could be based on AW rather than PBT, depending on the optimization target of the investment. Considering the AW, the optimal operational time corresponds to 5000 h (10,006 EUR/y) for the building without envelope insulation, 4000 h (5748 EUR/y) for the building with 5 cm of insulation and 4000 h (1914 EUR/y) for the building with 15 cm of insulation. The optimal operating time is close to 4000 h because it corresponds to the heating season duration in which a significant share of heat, produced by the CHP system, is used for both DHW and space heating. Considering the PBT, the optimal operational time corresponds to 8000 h both for the buildings without envelope insulation and for those with 5 cm of insulation (respectively, 6.9 and 9.2 y), and 5000 h for the building with 15 cm of insulation (14.7 y). As far as the configurations with high optimal operating time (i.e. 8000 h) are concerned, it has to be taken into account that the corresponding AW is quite low because considerable heat has to be discharge to the environment since the users require only DHW for almost half of the year. Figs. 11 and 12 show the incidence of a relative variation (20% e þ20%) on investment cost, biomass cost, feed-in tariff, cogeneration bonus and heat valorisation to NPV (Fig. 11) and PBT (Fig. 12). The graphs correspond to a 30 kWel CHP system operating 4000 h for the three analysed building configurations (0, 5, 15 cm of the envelope insulation). The feed-in tariff has been found to be the

insulation: 15 cm

insulation: 5 cm

40

0. 2

.1

60

0.1 0.1 5 0.0

5

0. 3 0.2

20 2,000

4,000

6,000

0.0 5

5

0.1 0.1

5

-0. 1

0

0

25 0.

Electrical Power [kW]

-0

80 0.2

0.2

0.25 0.3

-0.0

0.1 0. 1 5

8,000

2,000

5

0

0.2

4,000 6,000 Operational Time [h]

-0

0.05 0.1 0.15

8,000

0.15 0.2 0.25 0.3

2,000

-0.1 5

.05 -0.1

0 0. 0 5 0.1

-0.05

0.15

4,000

6,000

0 0.05 0.1

8,000

Fig. 6. Iso-PES curves depending on the power plant size and the operational time for three building configurations (0, 5, 15 cm of the thermal insulation of the opaque envelope).

158

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Fig. 7. Heat demand of the building without thermal insulation in the opaque envelope and heat generation by a power plant of 30 kWel operating for 4000 h per year.

Fig. 8. Discounted cash flows for a power plant of 30 kWel, considering three operational times (2,000, 4,000, 6000 h) and three building configurations (0, 5, 15 cm of the thermal insulation of the opaque envelope). Incentive regime with solid line and scenario without incentive with dotted line.

parameter that mainly influences both NPV and PBT while, the cogeneration bonus, which is related to the useful heat, has been detected as the parameter with the lower impact. These results confirm the need to update the subdivision of the subsidization, increasing the extent of the cogeneration bonus to promote the design of systems that properly valorise the heat share. This finding is also in agreement with the conclusions drawn by Noussan et al. [48] in a case study of a biomass-fired CHP system coupled with a district heating. Some detailed results about the energetic assessment are shown in Table 3 considering a power plant of 30 kWel and different operational time (i.e. 2,000, 4,000, 6,000, 8000 h). PES is greater than zero for all the three building configurations. For high

Fig. 9. Revenues from heat and electricity depending on the useful heat (both the revenues refer to the kWhel generated by the power plant). Incentive regime with dotted line and scenario without incentive with solid line.

operational time, the building heat demand covered by CHP is a considerable share but also the discharged heat is considerably high, in particular for the buildings with thermal insulation. These results highlight the complexities satisfying a large share of building heat demand and, having at the same time, a negligible share of heat discharged to the atmosphere.

4. Conclusions This work proposes an integrated approach for the assessment of the energy and economic performance of a biomass gasification CHP system installed in a residential block. This approach allows the estimation of the performance of the system in operation and provides indications for the design of systems energetically and economically efficient. For all the considered building configurations (12 alternatives), the PES analysis shows the possibility to set-up a biomass gasification CHP that allows a primary energy saving with respect to the separate production of heat and power. The primary energy saving depends on the heat discharged; the less heat is discharged, the larger is PES. A better exploitation of heat could be reached increasing the number of users that are served by the power plant but in that case a district heating system could be a more plausible scenario. At the current stage, partial or on/off operation of the gasification systems is not a feasible option due to management complexities to reach a steady state operation. Since the gasification power plant should run continuously, the heat discharged is strictly related to both plant size and thermal load profile of the final user. Therefore, the nominal power of the plant has to be considerably lower than the building peak load in order to limit the discharge heat. As a consequence, the heat demand of buildings has to be partially supplied by a back-up boiler, thus losing some benefits given by cogeneration. The gasification power plant, because of its

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Fig. 10. Annual Worth (AW) and Payback Time (PBT) for a 30 kWel power plant, considering different operational times and building configurations (0, 5, 15 cm of the thermal insulation of the opaque envelope).

Fig. 11. Incidence of the parameters (IC, biomass cost, feed-in tariff, cogeneration bonus, heat valorisation) on the NPV for a 30 kWel CHP system operating 4000 h for the three analysed building configurations (0, 5, 15 cm of the envelope insulation).

Fig. 12. Incidence of the parameters (IC, biomass cost, feed-in tariff, cogeneration bonus, heat valorisation) on the PBT for a 30 kWel CHP system operating 4000 h for the three analysed building configurations (0, 5, 15 cm of the envelope insulation).

Table 3 PES, disposed heat and heat by CHP system depending on the operational time (2,000, 4,000, 6,000, 8000 h) for a CHP system of 30 kWel. Oper. Time [hours]

PES

Disposed Heat by CHP Building heat demand covered by CHP

2000 4000 6000 8000 2000 4000 6000 8000 2000 4000 6000 8000

Building Configurations: Insulation: 0 cm

Insulation: 5 cm

Insulation: 15 cm

0.32 0.29 0.22 0.15 4% 14% 30% 44% 29% 52% 63% 67%

0.29 0.23 0.15 0.09 13% 29% 46% 56% 43% 70% 80% 87%

0.22 0.15 0.07 0.03 32% 46% 59% 66% 46% 73% 83% 92%

management complexities for the on/off operation, should be installed as base thermal load station. The economic analysis based on the power plant of 30 kWel shows there is a considerable influence of the useful heat on the discounted cash flow for both the scenario with and without incentive. The useful heat amount is related to the heat load profile, which varies for each building configuration. At the current stage of the technology, the incentive is essential for an economic return of the investment. The use of the heat share generated by a CHP system e even if it is based on renewable energy e is very important to promote high efficiency system in operation. With the current subsidization, the use of the generated heat is not a fundamental for the business plan. Such a subsidization distorts the energy sector promoting the electricity generation instead of the primary energy saving. A future development of this work will implement the integrated approach for energy performance assessment to district heating systems. Furthermore, different generation systems will be

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